Voltage doubler magnetic amplifier



y 8, 1951 R. E. MORGAN 2,552,203

VOLTAGE DOUBLER MAGNETIC AMPLIFIER Filed April 2, 1948 Inventor: Raymond E. Morgan,

His Attorney.

Patented May 8, 1 951 VOLTAGE DOUBLER MAGNETIC AMPLIFIER Raymond E. Morgan, Scotia, N. Y., assignor to General Electric Company, a. corporation of New York Application April 2, 1948, Serial No. 18,603

3 Claims. 1

My invention relates to saturable reactor magnetic amplifiers for amplifying small direct currents, and in particular to an improved regenerative magnetic amplifier, which provides an amplified D.-C. output signal which corresponds, in a substantially linear manner within the normal operating range of the amplifier, to the magnitude of a relatively feeble D.-C. input signal.

It is an object of my invention to provide an improved magnetic amplifier which will be responsive to extremely feeble direct current input signals.

It is another object of my invention to provide an improved magnetic amplifierin which the gain has been greatly increased without increasing the speed of response.

It is another object of my invention to provide an improved high sensitivity magnetic amplifier, the operation of which is not affected by the earths magnetic field.

It is a further object of my invention to provide an improved magnetic amplifier of the type described adapted for use with either high impedance or low impedance input signal circuits, which will not introduce undesirable voltages or currents into the input signal circuit.

The features of my invention which I believe to be novel and patentable are pointed out in the claims appended hereto. For a better understanding of my invention, reference is made in the following description to the accompanying drawing in which Fig. l is a schematic diagram illustrating a preferred embodiment of my invention, and Fig. 2 is an equivalent circuit which will be used in explaining the operation of Fig. 1.

Referring now to Fig. 1, alternating current input terminals I may be connected to a source of alternating current, such as a commercial 60 cycle electric outlet or an oscillator for producing alternating current of a higher frequency. A saturable magnetic core has a center leg 2 split lengthwise to divide the core into two similar sections 3 and 4, separated by a small air gap 5 which is preferably about inch to A; inch wide for reasons which are hereinafter explained. Each section has two legs, an outer leg and onehalf the center leg of the reactor, which preferably are of approximately equal cross-sectional area, so that fiux densities are equal in both legs of each section. Saturable magnetic core sections 3 and 4 have reactor windings B and i respectively wound thereon. Signal windings 8 and 9 are wound about both sections of the center leg 2. For many purposes, it is convenient to have two signal windings as shown, so that a bias signal can be applied to one winding and a varying signal to the other; or so that two signals can be applied to the two windings respectively to obtain an output proportional to the sum or the difference between the two. However, for many purposes one signal winding will be sufii' cient, and for other purposes more than two such windings will be necessary. The number of signal windings which may be placed on the center leg in the manner shown is limited only by the space available. A short-circuited winding 10 is also wound about both sections of the center leg. The purpose of this winding is discussed hereinafter.

Rectifiers II and I2 and capacitors l3 and 14 are connected, as shown, to form two similar circuits connected in parallel to terminals I, each of said circuits comprising a rectifier, a reactor winding, and a capacitor connected in series. The order of the elements is preferably that shown, but may be changed without greatly affecting the operation of the amplifier, provided only that the capacitors must remain at one end and be connected together. For example, the rectifiers may be connected in the circuit between the reactor windings and the capacitors. Output connections are provided to connect a load l5 across the two capacitors as shown.

Fig. 2 is an equivalent circuit of that portion of the circuit of Fig. 1 traversed by the alternating current supplied at terminals l. Windings 5 and I are shown as variable inductors, since the incremental or effective inductance of these windings changes as the magnetic core upon which they are wound saturates. It is evident that the circuit of Fig. 2 is essentially a voltage doubler circuit in which, if the impedance of inductors B and l is small, there is a D.-C. voltage across load I5 approximately equal to twice the A.-C. voltage at terminals I. If the impedance of inductors 6 and 1 is increased, a greater portion of the available voltage appears as a voltage -creased. windings 6 and I vary inversely with saturation rent through winding 6 has a D.-C. component 1 which produces a unidirectional magnetic flux flowing counterclockwise through magnetic core section 3,;:and:-the Dt-C. component of current flowing through winding I produces a unidirectional magnetic flux which fiows clockwise through magnetic core section 4. These two fluxes both pass through-signalwindings-B'and 9 in an upward direction and hence are of mutually additive polarity as they-pass through .these windings.

The unidirectional fluxes tend to partially saturate the saturable magnetic-{core sections '3 .and 4. Now if a direct current is passed through the signal windings. 8 or 9, this currentalso produces a unidirectional flux which is either additive or 'subtractiVe to the fluxes produced "by-winding's 6 and I-,-"depending'upon the directionof current-flow through the signal windings. -If the-:flux produced by current flowing in-the signal windings is of the additive polarity, thedegree of saturation of the magnetic core is increased,-but if this fluxis ofthe subtractive -'-polarity,' the degree of suclrsaturation is de- Since-' the effective inductances of of the magneticcore, currentsthrough the signal windings produce changes in the impedance of the-reactor windings-and hence changes in the voltage across load-l5. Within the normal operating range of"v the amplifier, changes in voltage across the load correspond in a substanr tially linear manner to the much smaller changes in current through the signal windings. This circuit is regenerative because any increase in saturation of the magnetic core, which reduces the effective inductance of the reactor windings, causeslarger: direct currents to flow through .5 the reactor windings, which in turn further increases the degreeoisaturation of the magnetic .half cycles; the fundamental frequency compo nents of alternating flux producedby these. two Hence, at any 1 windings are 180:out of phase. 'fgiven: instant the fundamental frequency flux --.-..-produced.'bywinding 6 flows through the signal eiwindingsjin aidirectionopp'osite tothat ofthe;

fundamental frequency flux produced by wind- 1."ffI-'hS8 two fluxes are thus of mutually subtractive...polarity :as' they flow throughthe ":"signal: windings, and consequently the voltages WhlChii'hGY induce iii-the signal windings cancel out andhaveno effectupon the input'circuits.

In high-sensitivity magnetic amplifiers previouslygused, it has frequently been necessary to wsurroundthe saturable-reactor element-with a "smagnetic shield, to prevent-the earths magnetic field, or other external fields, influencing the saturation of the core. It will be noted that in my amplifier, eVery flux path has equal density in opposite directions, since the flux flows upward through one leg and downward through another leg of equal cross-sectional area, so that efiects due to external fields cancel out and the earths magnetic field has no effect upon the operation of the amplifier.

In the circuit which I have described, the gain of the amplifier can be greatly increased without aiTecting the time constant of the input windings, which largelizgdetermines the time of responseof the amplifier, by-utilizing the resonant effects obtainable by proper choice of the values of capacitors I3 and It in conjunction .withtheinductance of windings 6 and I. These values of capacitance and inductance should be such thatxeach.winding and its associated capacitance is series resonant at a frequency somewhat -below-* the frequency of the alternat- 1 mg .curr.ent. -source connected to terminal I.

Then as the magnetic core saturates and the inductance of windings 6 and I decreases, each of the two circuits connected in parallel to ter- .minals 'i i .will'. approach: resonance;:and thefvoltage increaseacrosszloadi I ii'lwil'l'be. muchgreater 1; than itzw'ould be "were it'tnotforythee-mesonant effect. :Thecloser. to;resonancethercircuits are :tuned, the greater the: gainjwhichmayfbe realpromise value which must be chosen toefitsathe -particular application, and the? Voltage :;and frexquency stability: of zthe alternating current '7 source. 'With a: commercial'r60 cycle. alternating current source and a voltage variation not exceeding il0%, Iutilizeresonanteffectsto secure a gain l0' times the-maXimum-pbtainable :Without resonance. 1: If bothvoltage-andrirequency are held constantz-within 'i0;1%', I-can 1: secure good stability; withqmy, amplifier adjusttedto'give a'tpower' gain of'one billion. When the amplifier is; used "with a -60c'ycle 'per-zsecond source of alternatingcurrent," the reactor I windings and capacitors may be .initially'tuned core. This :regenerativeieffect makes it possible" 50 to a frequency as lowas 7 tolfi cycles per second; and still a considerable increase in gain is -obtainable due to resonant effects, since the inductanceofwindings 6 and I may vary over quite a widerange asthe magnetic core saturates.

'When this amplifier isused to amplify very feeble input signals, it is necessary that center 7 leg 2 be split lengthwise, as-described, in order 7 otherwise mask the applied signal.'

to eliminate the hysteresis efiect "which would If the core is not split, alternating components of fluxproduced by windings 5 and I tend to flow around the circumference of the magnetic core and not to flow through center leg 2 at all. This then leaves only unidirectional flux'flowing through center leg 2. -Because of hysteresis, a magnetic material does not return'to a state of zero magnetism'when the magnetizing force is removed. Hence there-tend to be a residual magnetism which may-be greater than the magnetization produced by a very feeble signal. I have found i that'with a non-split core, hysteresis limits the 'usable Di-C; signal applied to the signal windings to a minimum value of about 1 microwatt.

By splittin -the centerleg. lengthwise, a narrow :air gap is: provided which. has a "high reluctance and thus tends to prevent the flux following this circumferential path; so that alternating fluxes as well as unidirectional fluxes must flow through center leg 2. This insures that the magnetization of center leg 2 traverses a complete hysteresis loop each cycle of the alternating current, and so prevents residual magnetism from masking the signal. I have found that this split core construction may reduce hysteresis efiects by a factor of over a million. With my circuit, including the split core, I have obtained good results with D.C. applied signals as small as 1.0 micro-microwatt with an amplifler power gain of over one million.

The alternating flux through leg 2, however, will induce voltages in signal windings 8 and 9, unless the flux flowing through one section of leg 2 is exactly balanced by a flux 180 out of phase flowing through the other section of leg 2. If the signal windings are connected to high impedance circuits, and the signal windings have a large number of turns in order to obtain large gains and to make the amplifier responsive to very feeble signals, the voltages induced in the signal windings may be of the order of thousands of volts, which high voltages are dangerous to operating personnel and may cause the breakdown of the winding insulation. On the other hand, if the signal windings are connected to low impedance circuits, the induced voltages may cause circulating currents in the low impedance circuit which will very often be extremely undesirable. For example, if the signal circuit contains a thermocouple, circulating currents will cause internal heating of the thermocouple and thus produce an erroneous measurement of temperature. I have found that these difliculties are overcome by placing a short circuited winding Wound about both sections of center leg 2 as shown. The number of turns required in winding IE depends upon the number of turns in the other windings and other constants of the circuit; and it may, for example, be a single turn, or may in other cases be 100 turns or more. This short circuited winding presents a high impedance to any net alternating flux flowing through center leg 2, so that a flux which flows through one section of the center leg must be exactly balanced by another flux, 180 out of phase with the first, flowing through the other section. I have found that this arrangement, together with an air gap of proper dimensions as herein described, substantially eliminates the difficulties heretofore mentioned due to induced voltages and circulating currents in the signal windings.

I have found that a preferable width for air gap 5 is in the order of inch to inch. In most cases a. gap inch wide or greater is needed to prevent the objectionable hysteresis effects previously described. on the other hand, it is desirable that the gap be no larger than inch for the following reason: During a part of each A.-C. cycle, the flux density produced is very high. This high flux density, despite the action of shortcircuited winding l0, tends to produce circulating currents in signal windings 8 and Q and the circuits connected thereto. These currents, combined with the inductance of the signal windings, cause a phase shift of flux sufficient to cancel out small signals. That is, current circulates in the signal windings during that part of the cycle immediately after the rectifier has cut off current through the respective reactor windings, thus holding a flux that interferes with the signal. If

6 the air gap is about inch wide or less, some of this high density flux will cross the gap, and thus lower the flux density passing through center leg 2 sufliciently for short-circuited winding I!) to maintain balance between the fluxes in opposite sections of the leg.

In accordance with the provisions of the Patent Statutes, I have described the principle of my invention together with apparatus which I now consider to represent the best embodiment thereof; but I wish it to be understood that the apparatus described is illustrative only and that the invention can be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A voltage-doubler magnetic amplifier comprising alternating current input conductors, a saturable magnetic core, at least one signal winding and two reactor windings on said core, connections forming two circuits connected in parallel to said input conductors, each such circuit comprising one of said reactor windings, a rectifier, and a capacitor connected together in series, said capacitors being at corresponding ends of such circuits and connected together, each such circuit being series resonant at a frequency lower than the alternating current frequency, said rectifiers having such polarities that said two circuits are respectively conductive during opposite half cycles of the alternating current, said windings being so disposed that fundamental frequency alternating componentsof magnetic flux respectively produced by currents through the two reactor windings pass through said signal winding with mutually subtractive polarities and unidirectional components of such flux pass through said signal winding with mutually additive polarities, and output connections for connecting a load across the two capacitors.

2. A voltage-doubler magnetic amplifier comprising alternating current input conductors, a saturable magnetic core having a center leg split lengthwise to divide said core into two substantially similar sections separated by an air gap approximately 5 2' inch to inch wide, reactor windings on each of said two sections, at least one signal winding and a short-circuited winding each wound about both sections of the split center leg, connections forming two similar circuits connected in parallel to said input conductors, each such circuit comprising one of said reactor windings, a rectifier, and a capacitor connected together in series, said capacitors being at corresponding ends of such circuits and connected together, each such circuit being series'resonant at a frequency lower than the alternating current frequency, said reotifiers having such polarities that said two circuits are respectively conductive during opposite half cycles of the alternating current, said windings being so disposed that fundamental frequency alternating components of magnetic flux respectively produced by currents through the two reactor windings pass through said signal winding with mutually subtractive polarities and unidirectional components of such flux pass through said signal winding with mutually additive polarities, and output con nections for connecting a load across the two capacitors.

3. A voltage doubler magnetic amplifier comprising, alternating current input conductors, a saturable magnetic core, at least one signal winding and two reactor windings on said core, con- 'nections forming two circuits connected in parallel across said input conductors, each circuit comprising one of said reactor windings, a rectifier, and a capacitor .comiected together in series, said capacitors being atcorresponding ends of such circuits and connected together, each-circuit being series resonant at a frequency lower than the alternating current frequency, said rectifiers having polarities such that said two circuits are respectively conductive during opposite 510 Number The following references are of record 'in :the file of this patent:

UNITED STATES PATENTS Name Date 1,227,302 Osnos May 22, 19:17 2,052,978 Jester Sept. 1, 1936 2,084,117 Williams June 15, 1937 2,137,356 Schlesinger Nov. 22, 1,938 2,287,754 Barth June 23, 1942 2,309,156 Andrews Jan. 26, 1943 

